Currently, oxygen generators onboard the International Space Station produce oxygen by means of the electrolysis of water. Hydrogen, produced as a co-product of the electrolysis, is discarded overboard. Accordingly, large quantities of water are required to be transported to the Space Station, not only for the production of oxygen, but also for human consumption and other purposes, including hygiene. For long duration space missions at the Space Station or beyond low Earth orbit, the transportation of large amounts of water from Earth into space is prohibitive. Accordingly, water needed for the production of oxygen and other purposes must be supplied by a method other than transportation from Earth.
As astronauts consume oxygen through respiration, carbon dioxide is produced, which must be removed from the atmosphere within the Space Station or spacecraft designed for longer missions beyond Earth orbit. Currently, carbon dioxide generated in a spacecraft is removed by passing the cabin air through a bed of pelleted sorbent capable of adsorbing carbon dioxide. When the bed is saturated, it is regenerated by exposing the bed to space vacuum while heating the bed. The carbon dioxide that is liberated is then vented overboard. Clearly it would be beneficial to find a method of utilizing the carbon dioxide to produce life support consumables onboard of a spacecraft instead of venting the carbon dioxide overboard. Likewise, it would be beneficial to find a method for removing or utilizing carbon dioxide in extraterrestrial atmospheres that contain large quantities of carbon dioxide, for example, the Martian atmosphere.
Current interest is centered on the Sabatier reaction for in-situ resource utilization of carbon dioxide and for production of required human consumables, specifically, water and oxygen, for space missions. The Sabatier reaction specifically converts a mixture of carbon dioxide and hydrogen in the presence of a catalyst into a mixture of water and methane. The reaction accomplishes a primary goal of converting carbon dioxide, built-up in a space capsule or ubiquitous to an extraterrestrial environment, into valuable human consumables, specifically water, which is valued in itself or is converted into life-supporting oxygen. The Sabatier reaction eliminates the need to transport large quantities of water from Earth into space and, in this aspect, the Sabatier reaction reduces the payload projected from Earth and maintained on space voyages.
Methane resulting as a coproduct of the Sabatier reaction could be utilized advantageously as a propellant for a return voyage to Earth. On the other hand, methane is not easily compressed and stored in outer space; therefore, it may desirable to dump methane overboard or to convert methane into a high value liquid product, such as methanol, which is more readily stored.
The chemical equation for the Sabatier reaction for converting carbon dioxide and hydrogen into water and methane is presented in Equation 1:CO2+4H2→2H2O+CH4  (Eqn. 1)The chemical equation representing the electrolysis of water to produce oxygen and hydrogen is represented in Equation 2:2H2O→2H2+O2  (Eqn. 2)Human respiration takes the oxygen so produced and converts it into carbon dioxide per Equation 3:
                              O          2                ⁢                                            ->                              ->                                  ->                  ->                                                      ⁢                                                          Respiration                ⁢        C        ⁢                                  ⁢                  O          2                                    (                  Eqn          .                                          ⁢          3                )            The overall process of Equations 1, 2, and 3 provides for a closed loop among water, oxygen, and carbon dioxide. On the other hand, whereas one-half (50 mole percent) of the hydrogen needed for the Sabatier reaction can be derived from the electrolysis of water, the remaining one-half (50 percent) of the hydrogen needed for the Sabatier reaction will need to be obtained from Earth or other sources. It might be desirable to obtain the balance of hydrogen from the pyrolysis of the methane coproduced in the Sabatier reaction, per Equation 4:CH4+heat→C+2H2  (Eqn. 4)
The Sabatier process is discussed by James T. Richardson in “Improved Sabatier Reaction for In Situ Resource Utilization on Mars Mission,” published on-line in 1999-2000, at www.isso.uh.edu/publications/A9900/pdf/rich84.pdf. Richardson teaches the Sabatier reaction (1) over a catalyst comprising a pelleted bed of gamma-alumina having ruthenium deposited thereon, and alternatively (2) over a catalyst comprising a reticulated ceramic foam containing a gamma-alumina washcoat impregnated with ruthenium. Richardson notes that the Sabatier reaction is exothermic and subject to equilibrium conditions. He further teaches that above an operating temperature of 300° C., equilibrium conversions start to decrease.
The art also discloses a carbon dioxide removal and reduction system comprising a combination of a molecular sieve adsorbent bed for removing CO2 from a space cabin environment and a Sabatier reactor to convert the adsorbed CO2 into methane and water. See “An Advanced CO2 Removal and Reduction System,” by Gökhan Alptekin, et al., SAE International, 2004-01-2445 (2004), and “Prototype Demonstration of the Advanced CO2 Removal and Reduction System,” by Gökhan Alptekin, et al., SAE International, 2005-01-2862 (2005). The Sabatier section of the system is taught to be divided into a High Temperature Sabatier reactor and a Low Temperature Sabatier reactor, each one comprising a pelleted bed of catalyst in separate reactor housings. Both reactors are disclosed to be operated adiabatically.
Three publications disclose the Sabatier process using microchannel reactors for in-situ resource utilization of indigenous resources on Mars to produce hydrocarbon propellants. See the following: J. D. Holladay, K. P. Brooks, R. Wegeng, J. Hu, J. Sanders, and S. Baird in “Microreactor development for Martian in situ propellant productions,” Catalysis Today, 120 (2007), 35-44; Kriston P. Brooks, Jianli Hu, Huanyang Zhu, and Robert J. Kee, “Methanation of carbon dioxide by hydrogen reduction using the Sabatier process in microchannel reactors,” Chemical Engineering Science, 62 (2007), 1161-1170; and Jianli Hu, Kriston P. Brooks, Jamelyn D. Holladay, Daniel T. Howe, and Thomas M. Simon, “Catalyst development for microchannel reactors for Martian in situ propellant production,” Catalysis Today, 125 (2007), 103-110. The referenced microchannel reactor comprises a series of rectangular channels, the walls of which are lined with a porous intermetallic alloy felt coated with a catalytic metal on metal oxide washcoat, for example, ruthenium on titania (Ru—TiO2). The microchannel reactor further comprises a series of exterior channels through which an oil circulates countercurrently to the flow of the reactant gases, namely, carbon dioxide and hydrogen. Disadvantageously, the microchannel reactor provides for unacceptable heat transfer that leads to hot spots in the catalyst and catalyst deactivation. Moreover, the microchannel reactor provides an unacceptably high thermal mass and unacceptable mixing of reactant gasses between channels.
With respect to the above-identified art, a reactor bed of pelleted catalyst provides a disadvantageously large volume and payload mass for use in spacecraft. Having two or more pelleted bed reactor modules, as disclosed in Alptekin, et al., further negatively impacts the volume and weight of the system. More disadvantageously, beds of pelleted catalysts are prone to unacceptable heat transfer rates, sluggish transient response, and catalyst degradation over time, thereby rendering pelleted catalysts undesirable for space missions. A reactor based on a ceramic foam catalyst or the aforementioned microchannel design is disadvantageously limited to low throughputs due to a relatively high thermal mass. Additionally, local hot spots can occur in interior microchannels of the ceramic foam, which may facilitate catalyst deactivation.
Thus, while the utilization of carbon dioxide to produce life support consumables, such as water and oxygen, via the Sabatier reaction is an important aspect of the National Aeronautics and Space Administration's (NASA) Atmosphere Revitalization System (ARS) and In-Situ Resource Utilization (ISRU) concepts for low-earth orbit as well as long-term manned space missions, a need still exists in the art to find a compact, lightweight, and mechanically durable apparatus for the Sabatier process. The design is further challenged by the fact that the Sabatier reaction is highly exothermic (−165 kJ/mol). The art would also benefit from finding a catalyst for the Sabatier process that provides efficient conversion of carbon dioxide and excellent selectivity to water and methane with durability of catalyst performance. It would be more desirable if the apparatus and process were capable of operating at high throughputs and low operating temperatures while achieving essentially equilibrium conversion of carbon dioxide and equilibrium selectivity to water and methane.